This invention relates to a quench system for protecting superconducting magnets. High performance superconducting magnets operate at high current densities with zero or near zero resistivity. The high current densities and zero or near zero resistance minimize the size of the magnet winding, allowing for more compact magnets and increased magnetic fields.
If the temperature, field, or current is too high in some region of the superconductor, it loses its superconducting properties and becomes resistive or normal. Typically superconductors in the normal state are much more resistive than copper. The current flows in the high conductivity metal (usually copper or aluminum) that usually shunts the superconductor. The region of the winding where electrical current flows in the normal conducting material region due to the superconducting material losing its superconducting ability is called the quench zone or the normal zone. In the normal zone the winding is resistive and heat is generated by the current passing through it. In many instances this heating can result in local conductor damage. Although there is a normal conducting matrix (such as copper or aluminum) shunting the superconductor in superconducting windings, high performance magnets can generate large amounts of heating in this normal conducting material if the quenched zone is too localized. In order to prevent destruction of the magnet due to the strong local heating in the winding, it is necessary to quickly remove the stored magnetic energy from the magnet by dumping the stored magnetic energy in an external resistor or in an external electrical energy storage unit, or by depositing the energy relatively uniformly over the volume of the magnet to minimize the peak temperature of the initial normal zone.
High performance magnets are characterized by higher currents, current densities, as well as higher peak fields, forces and stored energy than other superconducting magnets. For high performance magnets it is difficult to remove the energy from the magnet fast enough, due to the high voltages that would be required. The high voltage is a result of the need to quickly discharge the magnet. It is more desirable to dissipate the magnetic energy in a substantial fraction of the volume of the magnet winding itself. The heating energy per unit volume required for achieving the superconducting-to-normal transition in the conductor depends on the nature of the magnet. Systems in good direct contact with liquid He require high energy inputs, on the order of 1 J/cm3. However, dry magnets that are cooled in the absence of liquid cryogens by direct thermal conduction to a cold anchor (for example, a cryocooler) require much less heating energy per unit volume, usually less than 100 mJ/cm3.
Superconducting windings may have a distribution of fields over the winding, such that some areas have large stability margins and others do not. In such instances a section of the winding may go normal, while the remainder stays superconducting, and as a consequence only a small portion of the magnet is heated. There may develop in such instances large temperature differentials across the winding, resulting in damage due to the build up of large mechanical stresses from the differential thermal expansion of the heated vs unheated areas. In this instance it is necessary to make the cold stable areas of the magnet go normal quickly to avoid such stresses that can also damage the winding.
To achieve internal dissipation (dumping) of the stored magnetic energy in a substantial fraction of the magnet volume, it is necessary to actively raise the temperature of a large fraction of the conductor winding above the current sharing temperature (the temperature at which current begins to flow in the normal conducting material) in a time that is small when compared with the natural decay time of the current. The heating makes the superconductor go normal, forcing the current to shift to the normal conducting material shunting the superconductor, and resulting in substantial heat dissipation in the bulk winding and a further overall temperature increase. The process results in more uniform temperature over the winding volume, and also decreases the peak temperature of the winding after the magnet has been discharged.
The heating of the conductor winding has been accomplished in the past by the use of local Joule heaters, either actively energized (by the use of external power supplies) or internally generated (by the use of internal loops that are inductively coupled to the main fields, energized by a current decrease in the main magnet or driven by internal transformers).
In low performance magnets, heating of the surface of the coil winding pack is sufficient because of the relatively long times allowed for the quench. Thermal diffusion through the conductor layers and insulation is high enough to allow heating of a substantial fraction of the coil cross section on a time scale short relative to the coil dump time scale.
In high performance magnets, in order to minimize the possibility of short-circuiting the coils and/or the heating elements, the Joule heating elements are located on the surface of the coil winding pack, usually in locations where the heating elements are not in direct mechanical load paths. In these high performance magnets, the times allowed for the dump of the stored energy are small enough so that it is usually not possible to depend solely on thermal conductivity across conductor layers or along the conductor to provide a relatively uniform temperature of the coil winding.
It is well known that the conductor windings, when in the superconducting state, can be heated by the use of AC losses (losses due the presence of an AC magnetic field). Several AC loss mechanisms are known to occur in superconducting windings, including eddy current losses, hysteresis losses, and coupling losses. Eddy current losses are caused by magnetic field diffusion through the normally conducting material (non-superconducting fraction). Hysteresis losses are due to magnetization effects in the superconducting material, as the AC field penetrates the surface of the superconductor. Coupling losses are due to losses through the superconductor/normal conducting material interface due to flux linkage through twisted superconductors.
The problem with heating the coil using AC losses by small rippling oscillation of the main magnetic field is that the power required to change the field is very high. If IDC is the current generating the main field, and IAC is the AC current, the ratio of the energy in the AC magnetic field to that in the main magnetic field is of order IAC/IDC, and this scaling results in very high powers being required in the externally driven AC magnetic field.
It is an object of this invention to heat a substantial fraction of the conductor of a superconducting magnet through non-conductive processes without requiring large reactive power to protect the magnet from destruction.